FIG. 1 depicts a schematic diagram of a portion of a typical wireless telecommunications system in the prior art, which system provides wireless telecommunications service to a number of wireless terminals (e.g., wireless terminals 101-1 through 101-3) that are situated within a geographic region. The heart of a typical wireless telecommunications system is Wireless Switching Center ("WSC") 120, which may be known also as a Mobile Switching Center ("MSC") or Mobile Telephone Switching Office ("MTSO"). Typically, Wireless Switching Center 120 is connected to a plurality of base stations (e.g., base stations 103-1 through 103-5) that are dispersed throughout the geographic area serviced by the system and to local- and long-distance telephone offices (e.g., local-office 130, local-office 138 and toll-office 140). Wireless Switching Center 120 is responsible for, among other things, establishing and maintaining calls between wireless terminals and between a wireless terminal and a wireline terminal, which is connected to the system via the local and/or long distance networks.
The geographic area serviced by a wireless telecommunications system is partitioned into a number of spatially distinct areas called "cells." As depicted in FIG. 1, each cell is schematically represented by a hexagon; in practice, however, each cell usually has an irregular shape that depends on the topology of the terrain serviced by the system. Typically, each cell contains a base station, which comprises the radios and antennas that the base station uses to communicate with the wireless terminals in that cell and also comprises the transmission equipment that the base station uses to communicate with Wireless Switching Center 120.
For example, when wireless terminal 101-1 desires to communicate with wireless terminal 101-2, wireless terminal 101-1 transmits the desired information to base station 103-1, which relays the information to Wireless Switching Center 120. Upon receipt of the information, and with the knowledge that it is intended for wireless terminal 101-2, Wireless Switching Center 120 then returns the information back to base station 103-1, which relays the information, via radio, to wireless terminal 101-2.
Indoor wireless telecommunications has been the subject of intense investigation in recent years for both voice and data communication. One particular area of investigation is how to ensure the adequate propagation of a wireless signal from a base station to a wireless terminal through a typical indoor environment. The walls, furniture and other objects in a typical indoor environment scatter the wireless signal and thus produce a complex multipath channel in which the signal propagation characteristics are substantially more difficult to predict than those in outdoor contexts.
Typically, the signal propagation characteristics are considered when designing and installing an indoor wireless telecommunications system. In particular, the signal propagation characteristics are advantageously considered when determining how many base stations are needed to provide coverage for a building and where in the building those base stations should be located. Because base stations are typically expensive to install and operate, it is advantageous to be able to determine how to provide the necessary coverage for the building with the fewest number of base stations. To do this, several techniques have been developed for modeling the propagation of wireless signals indoors.
One technique in the prior art for measuring and modeling indoor signal propagation is adapted from the power-law decay model used in modeling outdoor environments. The power-law decay model assumes that the base station's antenna is high above the ground and that there is line-of-sight propagation to the wireless terminal. In this case, the mean power, P, received at the wireless terminal decays in inverse proportion to the square of the distance from the transmitter, ##EQU1## up to some break-point. Beyond that breakpoint, the mean power at the wireless terminal decays in inverse proportion to the fourth power of the distance from the transmitter: ##EQU2## The location of the break-point is determined by the location at which the ground bounce signal interferes with the line-of-sight signal. For indoor environments, Equation 1 has been adapted to Equation 3, where .gamma. is fit to empirical trial measurements of the building of interest, in well-known fashion: ##EQU3## Although more sophisticated RF propagation models exist, the efficacy of any model is premised on empirical RF signal quality measurements that are accurately correlated with the position at which the measurements are made. Outdoors, an RF signal quality meter is used in conjunction with a satellite positioning system receiver (e.g., a Global Positioning System receiver, etc.) to take signal quality measurements that are correlated to the position at which the measurement is made. Indoors, however, it is typically difficult to gather RF signal quality measurements that are accurately correlated with the position at which they are made.
For example, FIG. 2 depicts a floor plan of an illustrative floor of a rectangular building, whose floor plan has been superimposed onto an x-y coordinate system. The building is 50 meters in the y-direction and 100 meters in the x-direction, and the floor of interest contains one wireless base station, base station 201, at location x=30, y=25, that radiates its signal with +20 dBm of power. Through experience, it has been observed that base station 201 is insufficient to provide satisfactory coverage throughout floor 200.
Typically, coverage is insufficient when there are areas in floor 200 in which the signal strength is below -65 dBm. A typical question is whether two similar base stations, one at x=25, y=50, and the other x=75, y=50, will provide at least -65 dBm of coverage throughout the building. Traditionally, this question would be answered by taking RF signal quality measurements of floor 200 and then answering the question based on the measurements and an indoor RF propagation model. Typically, however, the signals transmitted from a satellite positioning system are too attenuated by the walls of the building to be received by a satellite positioning system receiver, and, therefore, while it is easy to gather RF signal quality measurements in floor 200, it is problematic to accurately correlate them with position.
Therefore, the need exists for a tool that is capable of gathering empirical RF signal quality measurements and of accurately correlating those measurements with the position at which they are made.